For how many fake registrations are we talking about? I doubt they are real humans if many.
Is that a big and popular forum?
Having a security protection plugin goes beyond fake registrations or spam.
quite a big forum yes, see attached
i dont like using security plugins, most slow the site down... the server has good security... what security plugin do you recommend?
quite a big forum yes, see attached
i dont like using security plugins, most slow the site down... the server has good security... what security plugin do you recommend?
Ok, fair enough. So back to your main problem, you could try to block all bots and see if fake registrations continue and how many. I guess real human registrations is something you can not totally eliminate (easily).
Try this plugin:
https://wordpress.org/plugins/blackhole-bad-bots/
Previously i suggested a security plugin for your issue, because some plugins also check IPs against a database (cloud) and deny access. And because spammers (bots or human) are from KNOWN Ips, they will be blocked.
Spam and profiles are imho best to be blocked/prevented at login/registration and not AFTER.
There was a similar problem. They helped me solve it here. And in addition to captcha wpforo, I installed the Captcha by BestWebSoft plugin. Since then I have not seen a single spambot.